The invention relates to new and useful improvements in electomagnetically shielded housings for electric circuits. To ensure interference-free functioning of such circuits, the circuits must be shielded against electromagnetic waves. This means that the housing surrounding the electric circuit must comprise mostly metal parts. The shielding prevents electromagnetic waves from both entering and leaving the housing. This requires continuous electric connections, in particular contact spring elements, between the metal housing parts. These connections must have as few gaps as possible, in correspondence with the range of applicable electromagnetic frequencies.
German Utility Model G 92 09 519.4 discloses an RF-proof subrack having a cover with angled edge strips along the lateral edges of the cover. With the cover mounted, these edge strips engage in longitudinal grooves of two upper module rails of the subtrack. Inserted into each of the longitudinal grooves in this arrangement is a strip-shaped spring element. With the cover mounted and screwed against the upper module rails by means of fastening screws, this strip-shaped spring element produces an electric contact between the module rails and the cover.
A disadvantage is the use of separate elements for the electrical connection and mechanical connection between the module rails and cover, which complicates the mounting of the cover. Whereas the electrical connection is produced by the angled-off edge strips of the cover, which engage in the longitudinal grooves, the mechanical connection is provided by means of screws.
German Patent DE 39 28 461 C2 discloses an electromagnetically shielded subrack which has metal cover plates with edges that are angled off on all four sides. Profiled rails with grooves are fitted in the subrack and have a strip-shaped projection on one edge. V-springs are inserted into these grooves and extend along virtually the entire groove length. The angled-off edges of one cover plate, which extend in the width direction of the subrack, are pressed into the grooves during assembly. The angled-off edges press against the limbs of the springs and generate an electrical connection between the cover plate and profiled rails. The angled-off edges on the insides of the side walls rest on those ends of the cover plates which extend in the direction of the side walls. The side-walls can be screwed mechanically to the edges of the cover plates through holes drilled in the side walls at the level of the edges, and can be compressed to form an electrical connection.
A problem in assembling the subrack is caused by a counterpressure exerted by the V-springs on the angled-off edges of the cover plate. Thus, these edge regions of the cover plate bend upward, and must additionally be pressed onto the profiled rails for the purpose of assembly, causing extra work. It is therefore impossible to avoid subsequently screwing the cover plate to the side walls. Furthermore, despite screwing down the cover plate, it is impossible to prevent the angled-off edges from bending upward as result of the V-springs, in some cases causing the angled-off edges to project slightly. This can result in an undesirable permanent warping of the cover plate and an impairment of the electromagnetic shielding.
German Utility Model DE 94 02 714 U1, on the other hand, discloses a subrack having shaped strips and having top plates. The top plates are fastened via rivet joints and catch springs, whereby the catch springs latch on the shaped strips. For the purpose of electromagnetic sealing between the top plates and shaped strips, sealing elements in the form of foam plastic seals wrapped with metal foil are additionally fastened to the shaped strips.
A further subrack having module rails and having cover plates is described in German Utility Model DE 296 18 631 U1. In this case, a fastening groove, into which a spring element is inserted in a clamping fashion, is recessed in the module rails. An angled-off edge strip of the cover plate extends into the spring element. Contact is made with the cover plate via inwardly spread-out spring tongues of the spring elements.